pass entire format structure to various show_message functions

Various show_message* functions require formatting functions, which
were previously being passed individually as arguments.  Since we will
need to be needing to passing in more formatting function in the
future (ie. for crypto support), we here modify things so that we just
pass in the entire format structure.  This will make things much
simpler down the line as we need to pass in new format functions.

We move the show_format structure into notmuch-client.c as
notmuch_show_format.  This also affects notmuch-reply.c, so we create
a mostly-empty format_reply to pass the reply_part function to
show_message_body.